As a fixing device used in an image forming apparatus (e.g., a copying machine and a printer) using an electrophotographic printing method, a fixing device using a heat-roller fixing method is widely used. The fixing device using the heat-roller fixing method includes a pair of rollers (a fixing roller and a pressure roller) which press each other. Further, both or either one of the pair of rollers internally includes heating means containing a halogen heater or the like. The fixing device fixes a toner image on a recording sheet in the following manner. The heating means heats the pair of rollers to a predetermined temperature (fixing temperature). After that, a recording sheet on which an unfixed toner image is formed is carried to a pressure area (a fixing nip area) between the pair of rollers, and then the recording sheet passes through the pressure area. Thus, the toner image is fixed on the recording sheet due to heat and pressure applied thereto.
In general, a fixing device included in a color image forming apparatus uses an elastic roller. The elastic roller is a fixing roller having an elastic layer which is provided as a surface layer and is made of silicon rubber or the like. The elastic roller is used as the fixing roller, so that a surface of the fixing roller elastically deforms according to an uneven surface of an unfixed toner image and is in contact with the toner image so as to cover the toner image. This allows a color unfixed toner image whose toner amount is larger than that of a monochrome unfixed toner image to be favorably fixed.
Further, due to strain release of the elastic layer which occurs in a fixing nip area, it is possible to improve a releasing property with respect to color toner, which is more likely to offset than monochrome toner.
Furthermore, the fixing nip area has a nip shape protruding upward (i.e., toward the fixing roller side), that is, a so-called inverse nip shape. This makes it possible to more favorably separate a sheet from the fixing roller, thereby allowing the sheet to be separated without using any separation means such as a separation claw (self-stripping). This prevents insufficient image formation which is caused by the separation means.
In order to realize a higher processing speed, the color fixing device having the foregoing arrangement is required to have a greater nip width in the fixing nip area. As means for increasing a nip width, two methods are possible. One is a method for increasing a thickness of the elastic layer of the fixing roller, and the other is a method for increasing a diameter of the fixing roller.
Increasing the thickness of the elastic layer, however, causes the following problem: In a case where the fixing roller internally includes the heating means as in a conventional arrangement, the fixing roller including the elastic layer having an increased thickness does not increase its temperature according to an increased processing speed, because the elastic layer has a very low heat conductivity. Further, increasing the diameter of the fixing roller causes such a problem that (i) warm-up time is extended and (ii) much power is consumed.
In order to solve these problems, in recent years, a belt type fixing method as disclosed in Patent Document 1 is more and more used in the color fixing device. In the belt type fixing method, a fixing belt is provided between a fixing roller and a pressure roller so that the fixing roller and the pressure roller press each other via the fixing belt.
The belt type fixing device provides a short warm-up time, because the fixing belt having a small heat capacity is heated in the belt type fixing device. Further, with the belt type fixing device, it is not necessary to integrate a heating source such as a halogen lamp into the fixing roller. This makes it possible to increase a thickness of a low-hardness elastic layer made of sponge rubber and/or the like, thereby securing a large nip width.
As the belt type fixing method, Patent Document 2 discloses a fixing device (of a planar heat-generating belt type fixing method) having a planar heat-generating body as the heating means. In the planar heat-generating belt type fixing method, the heating means has a heat capacity smaller than that of a conventional heating roller. Further, the planar heat-generating body, which is the heating means, directly generates heat. This allows the fixing device to have improved heat responsiveness, compared with a conventional method in which a heating roller is heated indirectly by means of a halogen lamp. Thereby, the fixing device attains (i) a further reduction in the warm-up time and (ii) further energy saving.    [Patent Document 1]    Japanese Unexamined Patent Application Publication, Tokukaihei, No. 10-307496    [Patent Document 2]    Japanese Unexamined Patent Application Publication, Tokukai, No. 2002-333788
These fixing devices using the conventional planar heat-generating belt type fixing method, however, have the following problems.
In the arrangement of Patent Document 2, a shape of the fixing belt wound around the fixing roller is deformed at an area where the fixing belt is in contact with a heating member. This is because of the following reason: (i) The heating member, whose external shape is a substantially semicircle when viewed in an axial direction, is provided so as to be far from the fixing roller. (ii) A curvature radius of the heating member is larger than a radius of the fixing member. (iii) A width of the heating member is larger than a diameter of the fixing roller.
Therefore, the arrangement of Patent Document 2 has a problem with heat efficiency. The problem is caused as follows: Heat is emitted, to atmosphere, from an inner surface area of the fixing belt which area is not in contact with the planar heat-generating body or the fixing roller, and thereby the heat efficiency is decreased. Further, the arrangement of Patent Document 2 has a problem of slippage of the fixing belt. The problem is caused as follows: the deformed shape of the fixing belt wound around the fixing roller causes friction force to be applied to the fixing belt when the fixing belt slides on the heating member, and thereby the fixing belt becomes more prone to slip.
Further, during a stand-by mode in which the fixing roller does not rotate, pre-heating of the fixing belt is carried out only in an area where the fixing belt is in contact with the heating member. Therefore, when printing operation is started immediately after the stand-by mode, heat of the fixing belt is rapidly taken away by the fixing roller, and thereby a degree of undershooting is increased. As a result, it takes time for restoring a temperature of the fixing belt to a value which allows a toner image to be fixed. This arises such a problem that FCOT (First Copy Output Time) is extended.
Patent Document 2 describes that providing the heating member in the vicinity of the fixing roller reduces heat loss caused by the heat dissipation and improves belt running performance. However, with the arrangement where the heating member is provided so as to be far from the fixing roller, it is impossible to sufficiently reduce the heat loss and thereby to improve the belt running performance. Further, Patent Document 2 also describes that providing the heating member in the vicinity of the fixing roller allows the fixing roller to be heated by radiant heat and convection. However, with the arrangement where the heating member provided so as to be far from the fixing roller, it is impossible to sufficiently pre-heat a fixing roller having a large heat capacity.